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Contents

• Introduction
  • Polymer Molecular weights
  • Polymer Shape
• Polymer Synthesis
  • Addition Polymers
  • Chain Reaction Polymerisation
  • Condensation polymerisation

Introduction to Polymer Chemistry

by Fred Davis (Macro Group's Polymer Roadshow Officer 2004-2007, and lecturer at Reading University)

What is a polymer?

A polymer is a molecule made through the repeated addition of smaller monomer units. For example, poly(ethane) is made up from a series of ethene molecules.

In general a polymer chain can be made up from many hundreds of these monomer units.

Polymer Molecular weights

• Unlike small organic molecules, polymers are made up from a range of very similar chains with different molecular weights. Molecular weights for polymeric materials can range from a bout 2000 to over a million Da
• The molecular weights are very important since it is these which account for the properties of these systems
• Staudinger (Nobel Prize 1953) First suggested that polymers are long chain molecules and that their properties are determined by the intramolecular forces between these long chains

Polymer Size

• A polyethylene molecule may have up to a million units in the chain.
• If the polymer were stretched out straight then the chain length would be ca. 0.15 mm (C-C bond length is 0.15 nm).
• While enormous on a microscopic scale, this is very small on a macroscopic scale so other forces hold molecules together.
• For polyethylene these forces are mainly induced dipole -induced dipole in nature (sometimes referred to as Van der Waals forces), unless the material is crystalline (see later).
• At molecular weights up to 10,000, polyethylene behaves as a waxy solid.
• Lower molecular weights are needed to give good properties from polyesters (dipole-dipole interactions) and polyamides may become hard brittle solids at molecular weights as low as ca 2,000 (hydrogen bonding).

Polymer Shape

A series of identical monomer units implies that the polymer molecules exist as long linear chains. Indeed this is often the case and such materials are known as linear polymers. These materials will generally dissolve in solvents and become liquids when heated (although sometimes with some difficulty). When polymers behave in this way they are known as thermoplastics. However within this classification further property variations, which often depend on the way in which the polymer is processed. Three useful classifications are as follows:

Amorphous
In the liquid state (at high temperature) linear polymer chains flow freely and are arranged randomly. As an amorphous polymer is cooled the volume of the polymer contracts, but the random arrangement remains. If the material is cooled below a temperature known as the glass-transition temperature the polymer solidifies but there is still no order. This glassy polymer is extremely rigid and (if the polymer is colourless) is transparent.

Crystalline
Some polymers, particularly those with highly regular structures partially crystallize on cooling. Locally chains take up highly ordered crystalline structures, although crystallinity is never total. Crystallisation imparts particular strength to a polymer.

Fibres
Like crystalline polymers fibres are highly ordered, but here the chains are ordered by drawing them out in one direction. This drawing orientates the crystalline regions and imparts the high strength needed for fire formation. Polymers with strong intermolecular bonds such as nylons produce particularly good fibres.

Not all polymers are linear, and indeed not all thermoplastics are linear and some complex polymer geometries have evolved as shown in Figure 1. The polymers may be highly irregular (as in branched polymers) or highly regular as in dendrimers (click here for more information) and some cyclic systems

Some different polymer geometries

A polymer that is linked to a neighbouring chain is said to be cross-linked. A lightly cross-linked material is known as an elastomer. More heavily cross-linked polymers are completed rigid and are generally classified as thermosets.

Polymer Synthesis

Carothers and Flory classified polymers as addition or condensation polymers (the more usual classification nowadays is step reaction or chain reaction polymerisation), depending on the way the materials are synthesized. Addition polymers are usually made by the repeated addition of double bonds to a growing polymer chain.

Addition polymers

• Mostly addition polymerisation across carbon-carbon double bonds,
• Also includes some ring opening systems.
• Vinyl polymers are probably the most familiar polymers of this type.
• Examples include polyethylene, poly(vinyl chloride) poly-(styrene), polypropylene.

In an addition polymerisation unsaturated monomer units react together to form a polymer with essentially the same empirical formula as the monomer and with no side- products, usually the monomer is mono-functional so a general reaction will be:

Where X represents a range of groups such as chloro, phenyl, methyl, cyano etc.

Examples include the following:

Note that polymers often have a number commercial or commonly used names, these are given in square brackets after their proper (IUPAC) names.

Poly(ethene) [Polyethylene]:

Poly(propene) [Polypropylene]:

Poly(chloroethene):

Poly(phenylethene) [polystyrene]:

Poly(cyanoethene):

Poly(methylmethacrylate) [perspex®, plexiglass®]:

Chain Reaction Polymerisation

Chain reaction polymerisation

• involves an INITIATION step,
• followed by a PROPAGATION step
• finally, in some cases, a TERMINATION step.

Initiation

Initiation involves the generation of a suitable free-radical, (or other species such as a cation or an anion. Commonly this involves the thermal degradation of a peroxide to generate free-radicals.

Propagation

Termination (for free-radical systems)

Polymerisation of Conjugated Dienes

• The polymer contains predominately trans 1,4-units, the alkene units can be reacted to form cross-links.
• Synthetic rubbers can be lightly cross-linked diene-based polymers.

Not all addition polymerisations go via a free-radical mechanism some proceed with cation or anionic intermediates - an example of the later is provided by cyanoacrylate based systems which are the active constituent of "superglue"

Superglue

• Cyanoacrylates are particularly susceptible to anionic polymerisation by virtue of the stabilization provided by two electron-withdrawing groups.
• The polymerisation is readily initiated by rather weak nucleophiles including H2O and ROH.
• Thus, this material finds particular use as an adhesive, being polymerised by OH molecules that are generally present on surfaces ("super-glue").

Ziegler-Natta Polymerisation

Provides a route to forming high polymers especially of ethene and propene. In the case of poly(propene), this route allows control of the polymer stereochemistry. This can have an important effect on the polymer properties.

Polymer Tacticity

Isotactic

Syndiotactic

Atactic (Heterotactic)

• Isotactic Polymers are often partially crystalline (e.g. isotactic polypropylene)
• Syndiotactic polymers are sometimes crystalline
• Atactic polymers are rarely crystalline. (e.g. PMMA - polymethyl methacrylate formed by free-radical polymerisation)

Note polyethylene does not exhibit this type of isomerism.

Although most polymers are formed directly from monomer, it should be noted this is not always the case and polymeric systems may be reacted to form further products. Examples include the formation of carbon fibres from Poly(acylonitrile) or the formation of poly (vinylalcohol) from poly(vinylacetate).

Condensation polymerisation (sometimes called step reaction polymerisation)

In condensation polymerisation, reactive groups on both ends of each monomer react with one another. A growing chain also has a reactive group on each end and so the length of the chain can suddenly get a lot larger if two chains join together. Unlike addition polymerisation, incorporation of monomer into the polymer usually involves a change in empirical formula (loss of water or HCl).

The reactions that take place are usually ones familiar from low molecular weight organic chemistry, such as esterification (formation of a bond between an alcohol and a carboxylic acid), or amide formation. Clearly, two different reactive groups are required - reactions can be done with one monomer bearing two kinds of reactive group (A-B) or two different monomers (A-A and B-B).

Nylon

The reaction of an acid with an amine produces an amide, similarly, the reaction of a diamine with a diacid produces a polyamide. These polymers are particularly useful as fibres, because they can be drawn to form aligned chains linked together by hydrogen bonds. For example:

Aromatic polyamides form an important class of high strength materials known as aramids (click here for more information), while in nature amino acids form chain molecules called peptides or proteins (click here for more information)

Polyethylene Terepthalate (PET)

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